17.1 The Adaptive Immune System

• Immunity to certain infectious diseases can be acquired through exposure which is known as adaptive immunity.

• Vaccination (immunization): a procedure that harnesses the adaptive immune response.

  • A vaccine formulated with a harmless version of a pathogen incites an adaptive response which allows for people to become immune to illnesses without any danger of a full-blown infection.

• A crucial element is being able to differentiate between normal “self” cells and “nonself”.

• The adaptive immune system comes into play only when innate defenses such as skin fail to stop a microbe.

  • Primary response: the first time the adaptive immune system meets and combats a particular antigen.

  • Later interactions with that same cell or substance will cause a secondary response.

17.2 Dual Nature of the Adaptive Immune System

• Adaptive immunity is considered a dual system.

• Humoral Immunity: immune actions taking place in these extracellular fluids, brought about by protective molecules called antibodies.

• Antibodies (immunoglobulin) combat foreign molecules called antigens.

• T lymphocytes or T cells are the basis of cellular immunity .

  • T cell receptors that recognize an antigenic peptide attached to a specialized presenting molecule on a cell.

• Cellular immune responses focus on recognizing antigens that have already entered a cell.

17.3 Cytokine: Chemical Messengers of Immune Cells

• Cytokines are soluble proteins or glycoproteins.

  • Cytokines only acts on a cell that has a receptor for it.

• Interleukins serve as communications primarily between leukocytes.

• Chemokines induce leukocytes to migrate to areas of infection of tissue damage where they can begin to act against an infection.

• Interferons were originally named for one of their functions: the ability to interfere with viral infections in host cells.

• Hematopoietic cytokines help control the pathways by which stem cells develop into red  blood cells or different white blood cells.

  • Overabundance of cytokines can do significant damage to tissues which appears as a factor in the pathology of certain diseases such as influenza.

17.4 Antigens and Antibodies

• Substances that induce production of antibodies are called antigens.

  • Lipids and nucleic acids are usually antigenic only when combined with proteins and antibodies will react with the hapten independent of the carrier molecule.

• Antibodies are compact, relatively soluble proteins

• The number of antigen-binding sites on an antibody is called the valence of that antibody.

• Monomer is a bivalent antibody that has the simplest molecular structure.

  • The two sections that are located at the ends of the Y arms are called variable regions.

  • The stem of the antibody monomer and the lower parts of the arms of the Y are called the constant regions.

• The simplest and most abundant immunoglobulins are minors and can also assume some different sizes and arrangements.

• The five classes of Igs are IgG, IgM, IgA, IgD, and IgE.

  • IgG accounts for about 80% of all antibodies in serum.

  • IgM makes up 6% of the antibodies in serum and has a pentamer structure.

  • IgA accounts for only about 13% of the antibodies in serum, but it is by far the most common form in mucous membranes and in body secretion such as mucus, salvica, tears, and breast milk.

  • IgD makes up only about 0.02% of the total serum antibodies.

  • IgE constitute only 0.002% of the total serum antibodies and bind tightly by their Fc regions to receptors on mast cells and basophils.

17.5 Humoral Immunity Response Process

• Humoral immune actions take place in the extracellular spaces within the body.

  • B cells selected to mature can be activated.

• Clonal expansion is the process that allows activated B cells to produce plasma cells that make antibodies as well as memory cells.

• An antigen that requires a type of  cell called a T helper cell to activate a B cell is known as T-dependent antigen.

  • B cells can be activated directly by some antigens, called T-independent antigens.

• When an antigen-presenting cell makes contact with an antigen that can combine with its particular reception, the APC and antigen bind.

• The APC displays digested antigen fragments on its surface by surface by combining them with its major histocompatibility complex.

• T-independent antigens stimulate B cells directly, without the help of T cells.

• Harmful cells are usually eliminated at the immature lymphocyte stage by a process called clonal deleting.

17.6 Results of the Antigen-Antibody Interaction

• When an antibody encounters an antigen for which it is specific, their binding forms an antigen-antibody complex.

  • The strength of the bond between antigen and antibody is called affinity.

• The antibody molecule itself is not damaging to the antigen.

• The binding marks foreign cells and molecules for destruction or neutralization by pahgytes and complement.

• In agglutination, antibodies cause antigens to clump together.

• Opsonization is the coating of antigens with antibodies or complement proteins.

• Antibody-dependent cell-mediated cytotoxicity resembles opsonization in that the target organism becomes coated with antibodies.

• Antibodies may trigger activation of the complement system.

• In neutralization, IgG antibodies inactivate microbes by blocking their attachment to host cells.

17.7 Cellular Immunity Response Process

• Peyer’s patches are secondary lymphoid organs located on the intestinal wall.

• M cells take up antigens from the intestinal tract and allow their transfer to the lymphocytes and antigen-presenting cells of the immune system found throughout the intestinal tract.

• Antigen-presenting cells associated with cellular immunity include B cells, dendritic cells, and activated macrophages.

  • Dendritic cells have long extensions called dendrites.

• Macrophages are usually found in a resting state.

• T cells are classified by certain glycoproteins on their cell surface called clusters of differentiation or CD.

  • T cells can recognize an antigen presented on the surface of a macrophage and activate it, making the macrophage more effective in both phagocytosis and in antigen presentation.

• T regulatory cells make up about 5-10% of the T cells population.

• Cytotoxic T lymphocytes are not capable of attacking target cells as they emerge from the thymus as cells.

• On the infected cell’s surface, they carry fragments of endogenous antigens that are generally synthesized within the cell and are mostly of viral or parasitic origin.

17.8 Nonspecific Cells and Extracellular Killing by the Adaptive Immune System

• Natural killer cells can also destroy certain virus-infected cells and tumor cells and can attack parasites.

  • NK cells first contact the target cell and determine whether it expresses MHC class I self-antigens.

• WIth the help of antibodies produced by the humoral immune system, the cell-mediated immune system can stimulate natural killer cells and cells of the innate defense system, such as macrophages, to kill targeted cells.

17.9 Immunological Memory

• Antibody-mediated immune responses intensify after the primary response, where a particular antigen is first met, and corresponding antibodies are produced.

• The secondary response is due to the portion of activated B cells that, instead of transforming into antibody-secreting plasma cells, become memory cells.

• The intensity of the antibody-mediated humoral response can be reflected by the antibody titer, the relative amount of antibody found in the blood serum.
  

17.10 Types of Adaptive Immunity

• Immunity is acquired actively when a person is exposed to microorganisms or foreign substances and the immune system responds.

• Naturally acquired active immunity develops from exposure to antigens, illness, and recovery.

  • Naturally acquired passive immunity is the transfer of antibodies from a mother to her infant.

• Artificially acquired active immunity is the result of vaccination.

  • Vaccines introduce antigens to the body.

• Artificially acquired immunity involves the injection of antibodies into the body.

• When an individual is given artificially acquired passive immunity, it confers an immediate passive protection against the disease.